Bending properties of heat-treated NiTi strips for actuation of smart reinforced beams Raffaella Rizzoni, Mattia Merlin, Daniele Casari Department of Engineering, University of Ferrara, Italy E-mail: raffaella.rizzoni@unife.it, mattia.merlin@unife.it, daniele.casari@unife.it Keywords: shape memory alloy, nitinol, uniform bending, beam curvature. SUMMARY. Results of an experimental and analytical study on the free bending recovery of shape memory alloy strips are presented. Nitinol strips have been trained to memorize a bending shape and the best recovery performances have been obtained by heat-treating at 450 °C for 25 minutes. Free recovery has been observed during heating in an Ethylene Glycol based water solution and curvature data have been acquired by means of digital image analysis. To simulate free recovery, the Euler-Bernoulli beam model has been considered together with a one-dimensional phenomeno- logical constitutive equation for shape memory alloys. The calculated relations between the applied moment, the curvature and the temperature show good agreement with experimental data. 1 INTRODUCTION Shape memory alloys (SMAs) display a reversible crystalline phase transformation, enabling them to recover the original heat-treated shape (up to 6% strain) and to generate large recovery stresses when heated above a critical transformation temperature [6]. Due to their thermal stability, narrow hysteresis and corrosion resistance, NiTi wires or strips can be used in the design of actuators and sensors as well as functional structures in which they are directly or indirectly embedded in a polymer matrix. In literature, SMA actuators have been proposed for use in many engineering structures. Many studies have considered SMA wires embedded in a composite structure [4, 11, 12, 13]. Some non- embedding approaches have also been investigated. Baz et. al. [3] have studied the behaviour of SMA strips that have been thermally trained to memorise a bent shape, prestrained to a flat shape, and then inserted into the sleeves of a composite plate. On heating, the strips tend to recover the memorised shape and the plate is forced to bend. As in-plane displacements of the strips relative to the plate are allowed in this design, debonding and rapid ageing are prevented during multi-cycle actuation. In a previous work, the authors studied the actuation characteristics of NiTi strips working in bending and fixed to the surface of a polymeric plate. The preliminary experimental assessment of the heat treatment parameters that are needed to memorise a bent shape has been presented in [8]. Typical application of such a composite is the control of the geometry of blades to increase the performance of cooling fans in earth-moving engines [7]. This communication presents the results from related and ongoing research with the purpose of examining the characteristics of NiTi strip actuation in bending [9]. It focuses on the evaluation of curvature evolution during shape recovery on heating. The experimental techniques applied to characterise the behaviour of the SMA and to obtain its material properties are summarised in Sec- tion 2, where the mechanical properties and shape setting parameters are evaluated. They indicate that SMA strips show the best recovery performances when heat-treated at 450 °C for 25 minutes. To experimentally evaluate deformation during free recovery, the curvature evolution was estimated through digital image analysis during heating in an Ethylene Glycol based water solution. The ex- perimental data are presented in figure 4. Section 3 deals with the model that was developed to 1